As image sensors used for digital still cameras, digital video cameras, cell phone cameras, endoscope cameras, and the like, imaging devices, such as CCD sensors and CMOS sensors, have become widely known. These devices include a photoelectric conversion device having a pair of electrodes and a light receiving layer sandwiched between the electrodes and including a photoelectric conversion layer.
In order to improve photoelectric conversion efficiency of an organic photoelectric conversion device having such light receiving layer, the present applicant has proposed, in Japanese Unexamined Patent Publication No. 2007-123707 (Patent Document 1), an organic photoelectric conversion device that employs a mixed layer of a p-type organic semiconductor and a fullerene or a fullerene derivative (co-deposition layer of two materials) as a part of the light receiving layer.
According to Patent Document 1, a photoelectric conversion device having high photoelectric conversion efficiency with a favorable S/N ratio of photocurrent/dark current may be provided. Photoelectric conversion devices for use in sensors, imaging devices, and the like, however, the response speed is one of the important properties, as well as the photoelectric conversion efficiency and S/N ratio. It is, therefore, more desirable that the photoelectric conversion device described in Patent Document 1 is improved in the response speed.
Although the relationship between the response speed and film property is still unclear, the response speed may possibly be improved by improving unevenness in film quality arising from the difficulty in physical vapor deposition of organic thin films and control of the composition. Generally, in the physical vapor deposition of organic thin film, the organic material is likely to evaporate from near the wall of the heating boat that contains the evaporation source as organic substances have low thermal conductivities and the deposition rate is likely to vary due to bumping and collapse of the material within the boat, causing difficulty in stable evaporation. Thus, it is difficult to form photoelectric conversion layers having favorable properties for organic photoelectric conversion devices by physical vapor deposition at a high yield rate.
Japanese Unexamined Patent Publication No. 2011-091025 (Patent Document 2) discloses a vapor deposition material for flash deposition that allows formation of an organic photoelectric conversion layer at a stable deposition rate by optimizing the average particle size of particles used and homogeneity thereof. It is described that an organic EL device having a photoelectric conversion layer formed of such a vapor deposition material may prevent the initial drop in which luminance degradation occurs exponentially with time.
As a vapor deposition material suitable for forming a photoelectric conversion layer appropriate for an organic EL device by flash deposition, U.S. Patent Application Publication No. 20110175031 (Patent Document 3) discloses a particle formed of a host material of the photoelectric conversion layer and a dopant material strongly bonded together. According to such vapor deposition material, the compounding ratio of host material and dopant material is substantially constant so that a variation in composition that may occur at the time of supplying the materials may be prevented and a luminous layer having high in-plane luminance homogeneity may be formed.